The present invention relates generally to the field of semiconductor materials and, more particularly, to processes of forming semiconductor films on insulating substrates. Still more particularly, the present invention relates to silicon-on-insulator (SOI) processes and devices and, in its most limited embodiment, the present invention relates to silicon-on-sapphire (SOS) devices and processes.
The usefulness of thin, single crystal silicon films on electrically insulating substrates for electronic devices such as integrated circuits becomes apparent, when on considers that these circuits are contained within the top micrometer of the silicon material. In addition to utilizing the material more effectively, such devices cost less to manufacture. From the device standpoint higher circuit densities as well as improved device performance are obtainable in the areas of high speed signal processing, lower power consumption and higher tolerance in radiation environments.
SOI refers to those devices having a silicon thin film supported on an insulating substrate. SOI devices are becoming more important as the CMOS technology becomes the preferred technology for very large scale integrated circuits (VLSIC). Silicon-on sapphire (SOS) has been the most successful method of growing device quality silicon films on electrically insulating substrates. SOS structures consist of silicon films whose thickness can range from 0.1 to 0.5 micrometers on sapphire substrates. Silicon dioxide, as an insulator substrate, is preferable to sapphire. As an oxide of silicon it can be easily formed with the same purity of silicon. Further, it is more compatible physically and electrically with silicon being an oxide compound of silicon than sapphire which is an oxide compound aluminum. Devices with buried oxide insulating structures are made by forming a continuous oxide layer by implanting oxygen into the bulk silicon substrate. In these devices, the purpose of implanting oxygen is to create an insulator layer of silicon dioxide and cause in a thin silicon film to be electrically isolated from the bulk silicon substrate.
Although SOS has been an attractive materials system for use in the fabrication of VLSIC, the high defects structure of the silicon films has been regarded as a limiting factor in exploiting the full potential of SOS, particularly as film thickness is reduced to meet the requirements of advanced VLSIC's. Low channel mobilities, high leakage currents and a high number of interface states between the silicon film and the sapphire substrate are frequently cited problems. A significant reduction in the defect density within the silicon film of the SOS structure has been obtained by solid phase epitaxy and the simultaneous improvement in device performance has been reported. The silicon ion implant conditions preceeding solid phase epitaxy, however, may also damage the sapphire substrates if not properly selected. The presence of chemical, mechanical and structural defects at the semiconductor-sapphire interface has prevented the full exploitation of this technology. It is apparent then, that by improving this interface the performance of devices whose electrical characteristics are influenced by this interface will also improve.
The periodic nature of a single crystal lattice abruptly terminates at its surface. As a result, atoms located at the surface do not have a nearest neighbor at this surface causing their bond structure to be incomplete or dangle when compared to its inner lattice structure. For a single crystal semiconductor, one effect is to cause the introduction of states within the band gap of the semiconductor which are identified as surface or interface states. By chemically combining these surface atoms so as to form a compound the number of dangling bonds are reduced with concomitant reduction in surface states. The importance of silicon as a semiconductor is attributable to its oxide, silicon dioxide. The ability of silicon dioxide to stabilize the silicon surface is well known. It can in part be attested to by the fact that when present, silicon dioxide improves the silicon surface by reducing the silicon surface recombination velocity dramatically by orders of magnitude, down to a value of 1 cm/sec. Further, the silicon-silicon dioxide dioxide interface has a very low density of interface states. To date, no process or device has been disclosed in which an oxide has been implanted for the purpose of controlling interface or surface states or act as a diffusion barrier between the silicon and sapphire in an SOS or similar device.